Subscription type signaling system



Sept. 30, 1952 F. w. CROTTY ET AL SUBSCRIPTION TYPE SIGNALING SYSTEM 6Shets-Sheet 2 Filed June 10, 1949 Sept. 30, 1952 F. w. CROTTY ET ALSUBSCRIPTION TYPE SIGNALING SYSTEM Filed June 10, 1949 6 Sheets-Sheet 3$25 sw m ERWIN MROSCHKE FRANCIS W CROTTY IN V EN TORS THEIR AGENT Sept.30, 1952 F. w. CRQTTY ET AL 2,612,552

SUBSCRIPTION TYPE SIGNALING SYSTEM Filed June 10, 1949 6 Sheets-Sheet 4F ig. 4

WWW WW \/WV\I ERwiN M. RoscHK FRANCIS WCROTTY INVENTORS THEIR AGENT-Patented Sept. 39, 1952 NITED s'mrss SUBSCRIPTION TYPE SIGNALING SYSTEMFrancis W. Crotty, Elmwood Park, and Erwin M. Roschke, Broadview, llL,assignors to Zenith Radio Corporation, a corporation of IllinoisApplication June it, 1949, Serial No. 98,218

This invention relates to subscription type signalling systems, and moreparticularly to subscription type television systems.

In the copending application of Alexander Ellett et 2.1., Serial No.742,374, filed April 13, 1947, for Radio Wire Signalling System, U. S.Patent 2,510,046, issued May 30, 1950, and assigned to the presentassignee, there is disclosed and claimed a system for radiating acomposite television signal coded in accordance with a predeterminedcoding schedule and simultaneously transmitting over a second channelincluding a line circuit a key signal indicating the code schedule ofthe radiated composite signal.

The copending application of Erwin M. Roschke, Serial No. 773,848, filedSeptember 12, 1947, for Subscription Image Transmission System andApparatus, now -U'. S. Patent No.

2,547,598, issued April 8, 1951, and assigned to the present assignee,is directed to a particular type of subscription television systemwherein the time relation between the video-frequency signals and thesynchronizing signals of the composite tele vision signal is alteredduringspaced time intervals to provide coding.

It is an important object of the present invention to provide a novelsubscription type television system in which coding is accomplished in amanner different from that utilized in the abovementioned copendingapplications.

The present invention provides a subscription type of television systemincluding a transmitter and a receiver' The transmitter comprises avideo-frequency signal generator, such as an iconscope or an imageorthicon, and an associated scanning system for controlling thevideo-frequency signal generator to develop during recurrent traceintervals videofrequency signals representing a scanned subject andindividually having a predetermined maximum amplitude range. Thescanning system also includes a timing-signal generator for developingduring interposed intervals timing signals normally having a fixedamplitude with respect to the amplituderange of the video-frequencysignals. A mixer device is coupled to the video-frequency generator andto the scanning system to produce a composite television signal whichincludes in alternation the video-frequency signals and the timingsignals. Coding apparatus is coupled to the mixer device and develops acoding signal for altering during spaced time intervals the amplituderelation between the maximum ampltiuderange of the video-frequencysignals and the amplitude of the timing signals, thereby to code thecomposite signal. A key-signal generator is provided for developing akey signal indicating the times of occurrence of the spaced intervalsduring which coding is efiected. The transmitter is also provided withan antenna or other meansfor radiating the coded composite signal.

11 Claims. (Cl. 178-51) The subscriber receiver includes animagereproducing device and an associated scanning system, and a linecircuit coupled to thekeysignal generator at the transmitter extends tothe receiver for supplying the key signal thereto. Decoding apparatus isprovided in the receiver, and this apparatus is responsive to the keysignal received over the line circuit to develop :a decoding signalwhich acts conjointly with the-coded composite signal received over theair to cause the image-reproducing device and its scanning system totrace an image of the scanned subject.

The features of the present invention which are believed to be novel areset forth with particularly in the appended claims. The invention,together with further objects and advantages thereof, may bestbeunderstood, however, by reference to the following description takenin connection with the accompanying drawings, in

the several figures of which like reference nu morals indicate likeelements, and in which:

, Figure l is a schematic block diagram of a subscription typetelevision transmitter constructed in accordance with the presentinventicn,

Figure 2 is a schematic block diagram of a subscription type televisionreceiver constructed in accordance With the invention,

Figure 3, 5 and 7 are schematic circuit diagrams of components of thetransmitter and receiver of Figures 1 and 2, and

Figures 4 and 6 are simplified graphical representations which areuseful in explaining the operation of the invention, theserepresentations not being drawn to scale.

Figure l is a schematic block diagram of a subscription type televisiontransmitter embodying the present invention. The transmitter comprises avideo-frequency signal generator l!) which includes a lens system and apicture-converting device 12,, such as an iconoscope or an imageorthicon. Synchronizing-signal and sweep generators l3 providehorizontal and vertical sweep signals for the horizontal and verticaldeflection coils l4 and 15, respectively, for controlling generator Itto develop during recurrent trace intervals video-frequency signalsrepresenting a scanned subject. Video-frequency signals produced bypicture-converting device it and its associated scanning system arepassed through a video amplifier It to a, synchronizings'ignal andpedestal mixer device ll, Where they are mixed with timing signalsreceived from generator is over leads it to provide a compositetelevision signal Which includes in alternation'the video-frequencysignals from device l2 and the timing signals received from leads l8.Thus, the output of mixer device ll appearing on leads [8 representsasignal identical with that which, after modulation on a radio-frequencycarrier divider 2,0.

ll, 1948, for Random Frequency Dividers]? now U. S. Patent No.2,588,413, issued March 11, 1952, whichapplication is assigned tothesame assignee as the present application.

Frequency divider 26 .operates to pass selected verticalsynchronizing-signal pulses to a keysignal' generator 22' over leads 23.Key-signal generator '22 may "be, for example, of the type "shown anddescribed in the aforementioned application of Alexander Ellett et al.and generates a key signal comprising bursts of superaudible- ,frequencyenergy," the occurrences of which are determined by the pulses passed byfrequency Vertical synchronizing-signal pulses from leads "2|,horizontal blanking voltage pulses by way of leads 24 from generator 13,and the key signal from leads 25 coupled'to key signal generator 22 areall supplied to a coding device 26, the-construction of which is to behereinafte described in detail. The uncoded composite television signalfrom leads 19 is applied to coding -apparatus 26, where it is coded inaccordance with a random coding signal developed therein in response tothe vertical synchronizing-signal pulses and to the key signal, positetelevision signal is'impressed on a carrierwave generator and modulator21 by way of leads "28, and the resulting radio-frequency wave,

modulated in accordance with the coded composite television signal, isradiated by means of anantenna29.

The key signal from generator 22'is impressed on a line circuit'3flextending to a central station 3| for distribution to authorizedsubscribers over lines 32,33, etc. l

Video-frequency signal generator l0, synchronizing-signal and sweepgenerator l3, video amplifier i6, mixer device I1, and carrier-wavegeneratorand modulator 2-1 mayall be of conventional construction.

In accordance with thepresent invention, the coding signal developed incoding apparatus 26 is utilized to alter the maximum amplitude-range ofthe video-frequency signal relative to the amplitude of the interposedtiming signals. This alterationis accomplished at randomly selectedspaced time intervals to provide a maximum of secrecy.

- Figure "21s a schematic block diagram of a subscription typetelevision receiver for providing decoded reception of the televisionsignal radi- 4 signal separator 45. Vertical synchronizing-signal pulses:from separator 45, appearing on leads 46, are employed to drive avertical sweep generator 41 to provide scanning signals for the verticaldeflection coils 48 associated with an image-reproducing device orpicture tube 49. Horizontal synchronizing-signal pulses from separator'45 are applied to an AFC (automatic frequency control) phase detector50 for phase comparison with signals from a horizontal oscillator '59.Horizontal oscillator 59 drives a horizontal sweep generator 5| toprovide scanning signals for the horizontal deflection coils 52associated with image-reproducing device 49.

The detected video-frequnecy signals from video detector 43 areimpressed on a decoding device 53 by wayof leads 54. Decoding appara-32.- The decoding signal generated in apparatus The coded comstages ofradio-frequency amplification 42 may I be provided before theoscillator-converter 4|.

pulses are derived from the detected output of video detector 43 bymeans of a synchronizing- 53 is utilized conjointly with the detectedvideofrequency signals on leads '54 to provide a decoded compositetelevision signal on output leads 55. The decoded composite signal,after amplification in -a videoamplifier 56, is impressed on theinputcircuit ofimage-reprod-ucing device 49.

In the illustrated embodiment, central station 3| is presumed to be acentral telephone switchboard, and. line 32 may conveniently be anexisting telephone line. In order to enable a subscriberqto. use histelephone handset 51 during the receptionpf a key signal on line 32, aspeech pass filter 583s inserted between handset 514 and the line. It isto be clearly understood, however, that the invention is not to belimited to the use of telephone circuits .as the key-signal channel;

itis contemplatedthat other existing distribution systems, $11,611 asthe power lines, may be employed for this-purpose.

'Those'portions'of'the transmitter and receiver of Figures 1 and-'2which are'required to transmit andreceive the accompanying sound-signal,com ponent "form no part of the present invention andare therefore notshown.

Coding apparatus'26 of the transmitter of Fi ure 1 may lee-considered asbeing composed of two portions. is to produce a control signal thecharacteristics of which change in response to the nextsucceeding'vertical synchronizing-signal pulse following theinitiationyof a key signal burst, and also in response to the nextsucceeding vertical synchronizing-signal pulse after the termination ofsuch burst. This first portion is connected to the keysignal generator22 by means of leads 25, and to the synchronizing-signal and sweepgenerator l3 'by way of leads 2'4. The function of the second portion ofthe coding apparatus is to alter the maximum amplitude-range of thevideo-frequency signal'relative to the amplitude of the timing signalsduring intervals determined by the cen-trolsignal. The externalconnections of the second- 'portion, therefore, extend to the horizontalblanking voltage generator of device 13 by way of leads 2!, and to thesynchronizing-signal andpedestal-mixer II by way of leads IS. The codedriomposite television signal then appears onleads-ZB -and is supplied tocarrier-wave generator and modulator 21.

Thus, the first portion of coding apparatus 26 actuates thesecondportion during spaced time intervals determined by random frequencydivid- The function of the first portion nected to ground.

er 2!! and key-signal generator 22, during which intervals the amplituderelation between the components of the composite signal is to bealtered. The second portion of the coding apparatus, in response to suchactuation, generates a coding signal which is superimposed on theuncoded composite television signal to provide a coded composite signalwhich may be utilized only by authorized subscribers concurrentlyreceiving the appropriate key signal to reproduce a clear image of thescanned subject.

The first portion of the coding apparatusfit,

of the secondary winding t l of transformer 53 is coupled to the controlelectrode E5 of an electron-discharge device 65 by means of a couplingcondenser 5i. The other terminal of secondary i winding 64 is connectedto ground through a resistor 58. Control electrode 535 is connected toground through a grid resistor The cathode if of discharge device at isconnected to ground by means of a cathode bias resistor l i, whichresistor is shunted by a bypass condenser E. Cathode is is alsoconnected to the positive terminal of a sou so '13 of unidirectionaloperating potential. here shown as a battery, through a resistor it. Thenegative terminal of source E3 is grounded. The anode E5 of device 86 isconnected to the positive terminal of source I3 through a load resistori5. Anode i5 is also coupled to a rectifying device ll by means of acoupling condenser '73. The other side of rectifying device at isconnected to ground through the parallel combination of a resistor I9and a condenser 8t.

Regenerative feedback from the output circuit to the input circuit ofdevice is provided by means of a transformer iii connected therebetween.One terminal of the primary winding 82 of transformer 8! is directlyconnected to the junction between condenser iii and rectifying deviceTi, and the other terminal of the primary winding 82 is coupled to thisjunction. through a condenser 83 and is connected directly to ground.One terminal of the secondary winding a l of transformer 8! is grounded,and the other terminal or" this winding is connected to the junctionbetween resistor 58 and secondary winding 54 of input transformer t3through a resistor 35.

The control electrode of a second electrondischarge device Bl isconnected to the junction between rectifying device i? and resistor isby means of a coupling resistor 88. The cathode 39 of device 8'5 isdirectly connected to cathode Id of device so. The anode 9B of device8'! is con nected to the positive terminal of source 13 through a loadresistor ti.

A pair of vertical synchronizing-signal pulse input terminals 92 and &3are also provided, these terminals being connected to lines 2i of thetransmitter of Figure 1. Terminal 92 is directly con- Terminal 93 iscoupled to control electrode 86 or device 8! by Ineansof a resistor ccand a condenser 95 connected in series,

and terminal as is also connected to anode so of *6 device 81 through aseries-connected resistor 56 and condenser 91. Anode is connected toround through a resistor 93.

Anode 90 of device 8'! is coupled to the control electrode 99 of anelectron-discharge device its by means of a coupling condenser I 31.Control electrode 99 is connected to ground through a grid resistor I82.The cathode I83 of device let is connected to ground through a cathoderesistor IM. The anode I05 of device IfiD is connected to the positiveterminal of source 73 through a load resistor I86. Anode I05 is alsocoupled to the control electrode I0! of an additional electron-dischargedevice Hi8 by means of a coupling condenser I09. The cathode lie ofdevice I08 is directly connected to cathode E03 of device I 00, andcontrol electrode It! is connected to cathode III] by means of a gridresistor III. The anode II2 of device 563 is connected to the positiveterminal of source '53 through a load resistor I I3.

One output terminal H4 is connected to control electrode 99 of deviceillil through a balancing resistor H5, and the other output terminal II6 is directly connected to ground.

The operation of the circuit of Figure 3, representing the first portionof coding apparatus 26 of the transmitter of Figure 1, may best beunderstood by reference to Figure 4. Key-signal bursts, shown in curveA, are impressed between terminals 60 and 6| of the circuit of Figure 3.i

As previously stated, these bursts are initiated by randomly selectedvertical synchronizing-signal-pulses and are utilized to determine thespaced time intervals during which, the ampli tude relation between thevideo-frequency signals and timing signals of the composite televisionsignal is to be altered. In the illus trated embodiment, the key signalbursts are of constant duration determined by the circuit parameters ofkey signal generator 22. By initiating the change in transmitter modeoperation by vertical synchronizing-signal pulses, the mode change iseffected at the transmitter during vertical retrace intervals to avoiddistortion of the reproduced image which would result if the modechanges were initiated at other times. As pointed out in theabove-mentioned Roschke application Serial No. 773,848, each burst ofkey signal is transmitted over the line circuit one field period beforethe corresponding change in mode in order to allow for delays whichusually occur in the line circuits. In order to simplify the explanationof the present invention, a condition of zero delay is assumed.

The key signal bursts are amplified by device "56. The amplifier is maderegenerative by supplying a portion of the output signal back to theinput circuit by means of transformer 3i to prevent noise signals andthe like from aiiecting the coding apparatus. Device 66 is maderesponsive only to signals above a certain threshold level by means ofthe'voltage divider action of resistors I l and 'EI connected betweenpotential source it and ground. The amplified key signal bursts fromdevice 65 are rectified by device l7, and negative polarity pulsesindividually of duration equal to that of the individual key signalbursts, as represented in curve B, appear across resistor L3.

The vertical synchronizing-signal pulses de rived from generator I3 ofFigure l are shown in curve C, and these pulses are impressed betweenterminals 92 and 93 by means of leads 2i. Keysignal burst I I it ofcurve A is initiated by a vertiacrajsse v.2, and the circuitcharacteristics are such that in the presence of each rectified keysignal burst, device 01 is non-conductive and the verticalsynchronizing-signalpulses are nottransl'ated by device 1'. However, inthe intervals between the rectified key-signal bursts, device..81 isconductive and'the positive polarity vertical synchronizingsignal pulsesappliedi tocontrol electrode 86 are amplified and supplied; to thecontrol electrode 99 of device I00, as negative polarity pulses of twicetheir originalamplitude. Howevezypositive polarity verticalsynchronizing-signal pulses are continuously supplied to controlelectrode 99 by means of resistor 96 and condenser 31, and thesepositive polarity pulses are superimposed on the doubleamplitudenegative polarity pulses sothat the resulting signal impressed oncontrol electrode 99 comprises positive polarity pulses in the presenceof the rectified; key signal bursts and negative polarity pulses ofequal amplitude in the absence of key signal bursts. This signal isrepresentedin curve D of. Figure 4.

Devices I00 and I00 are connected as a conventional single-shotmultivibrator circuit arranged so'that-device I 08 is normallyconductive. The multivibrator is stable'in this condition in the absenceof an input signal. The time constant of condenser I00 and resistor IIIis made longerthan the duration of anyindividual key signal bursts andalso longer than the longest interval between two successive key signalbursts.

1 Consequently, the multivibrator is maintained in its stableconditionuntila positive polarity pulse,

- such as pulse 1 I22 of curve B, is impressed on control electrode 09to render device 100 conductive. The multivibrator then trips and devicel'IlIS becomes non-conductive. This secondarycondition is maintaineduntil such time as a negative polarity pulse, such-as pulse H24 of curveD, is impressed on control electrode 09 to out off device I00 andrestore the multivibrator to its normal condition. I

The output of the multivibrator,appearing between'te'rminals 'I I4, andI I6, is then represented by curve E of Figure 4. This output isutilized as ,a control signal to actuate :the second portion of thecoding "apparatus; and the; amplitude of the control signalchanges attimes-determined by the vertical synchronizing-signal pulses immediatelysucceeding the initiation and terminacharge device I 2I through avariable resistor I22, and terminal H0 is grounded. Cathode I isconnected to ground through a cathode resistor I23. The controlelectrode I24 of device I2I is directly grounded, and the anode I25 ofthat device is connected to the positive terminal of a source I28 ofunidirectional operating potential, here shown as a battery, through aload resistor I21. The negative terminal of source I20 is grounded.

.An additional pair of input terminals I28 and 5129, for receivinghorizontal blanking voltage pulses-from leads 24' of the transmitter ofFig ure-l, are connected 'in'the input circuit of an Teradditional"electron-discharge device I 30. minal I28 is connected to the controlelectrode I32I of device I30, and terminal I29 is grounded. The cathodeI32 of device I is grounded, and

the anode 133 of that deviceis connected to the positive terminal ofsource I25 through load resistor' I21. Anode I33-of device I30 is alsodirectly connected to anode I25 of device I2I.

. Anode I33 of device I30 is coupled to the control electrode I34 of anelectron-discharge device I 35 by means of a coupling resistor I36.

Control electrode I34'is connected to the negative I terminal of aunidirectional biasing potential source I31, here shown as a battery,through aresistor I38. The positive terminal of source I31 is grounded.The cathode I39 of device I35 is connected to ground through a cathodeload resistor I40. The anode I4I of device I35 is dircctly connectedtothe positive terminal of source I26.

A further pair of input terminalsi42 and I43 are connected in the inputcircuit of another 1 grid resistor I48 is connected between control 1electrode I45 and cathode I41. The anode I49 of device I44 is directlyconnected to the positive terminal of source I26.

A pair of output terminals I50 and I5I are connected-in the commonoutput circuit of devices I35 and I44. Terminal I50 is directlyconnected to cathodes I39 and I41, and terminal I5I is grounded. I

The operationof the second portion of the coding apparatus, shownschematically in Figure 5, may best be understood from a considerationof that diagram taken in conjunction with the waveforms of Figure 6.

Devices I35 and I 44 are connected with a common output circuitcomprising cathode resistor I40. In the normal state, device I35 isbiased to cut-off by potential source I31, and no anode current fromdevice I35 flows through cathode resistor I40. Uncoded compositetelevision signals represented by curve H (Figure 6), applied betweenterminals I42 and I43 from leads I0 of the transmitter of Figure 1, aretranslated by device I44 and appear'unaltered across output resistor I40and hence between output terminals E of Figure 6 having been expandedwith respect to that of curve E of Figure 4. Electrondischarge deviceIZ-I of Figure 5 is biased to cutoil at a voltage E1 intermediate themaximum positive and negative values of the control signal voltage.

maximum negative potential, device I2! is rena dered conductive, and thepotential at the anode During the intervals between .key signal bursts,when the control signal is at its voltage pulses.

I 33 of device I30 is low Thus, during the intervals between key signalbursts when the control signal is at its maximum negative potential,device I35 remains in a non-conductive state, and the compositetelevision signal is translated unaltered to the output terminals I50and II.

When the control signal is at its maximum positive potential, device I2Iis rendered non-conductive and the potential appearing at anode I25 isincreased, thus tending to counteract the negative bias potential fromsource I31 and tendin to render device I35 conductive. In this state,device I35 is placed under the control of incoming horizontal blankingvoltage pulses from generator I3 of the transmitter of Figure 1. Thesehorizontal blanking voltage pulses, represented by curve F of Figure 6,are applied to the input circuit of electron-discharge device I30, whichis biased to cut-off at a voltage E2 intermediate the maximum positiveand negative potentials of the blanking voltage pulses. In the presenceof a blanking voltage pulse, such as pulse H52 of curve F, device I38 isrendered conductive, and the potential at anode I33 drops. Thispotential drop tends to counteract the potential rise due to the cut-offof device I2I and again biases device I35 to cut-off by reason ofpotential source I37. However, during the intervals between blankingvoltage pulses, for example interval I I33 of curve F, device I35 isrendered non-conductive and the potential at anode I33 remains high.This renders device I35 conductive, and produces a voltage component,represented by pulse H35 of curve G, across cathode resistor MB due tothe anodecurreht or" device I35. This-pulse is effectively superimposedon the video-frequency signal component of the composite televisionsignal and results in a shift of the maximum ampli-= tuderange of thevideo-frequency signals relative to the amplitude of the horizontalblanking Thus, the coded composite tele.

ision signal appearing between output terminals I59 and I5I is oithetype shown in curve J of Figure 6. I

The coded composite television signal appearing between terminals I53and I5I is then supplied to the carrier-wave generator and modulator IIIof the transmitter of Figure 1, and the modulated carrier wave issubsequently radiated by means of antenna 29.

In order to obtain decoded image reproduction at an authorized receiver,it is contemplated that the subscriber must place a telephone call to acentral station to request an appropriate key signal. The telephone callis necessitated by the practical consideration that the subscriber mustbe charged for utilizing the key signal. At an authorized subscriberreceiver, such as that shown schematically in Figure 2, the codedcomposite television signal is intercepted, heterodyned, detected, andpassed through decoding apparatus 53 in order to supply a decoded signalto imagereproducing device ie to enable that device and its associatedscanning system to trace an image of the scanned subject.

The decoding apparatus 53 of the receiver also comprises a, firstportion and a second portion. The firstportion may be identical with thefirst portion of the coding apparatus used at the transmitter, and thisportion of the decoding apparatus operates in the manner shown anddescribed in connection with Figures 3 and 4. The'function of the firstportion of the decoding apparatus is to provide a control signal at thereceiver which is in synchronism with the control signal used at thetransmitter to provide coding of the composite television signal. Thiscontrol signal, represented by curve E of Figure 4, is developed by thecircuit of Figure 3 and appears between output terminals IM and IIS.

The second portion or" the decoding apparatus 53 is illustrated inschematic form in Figure 7. In Figure. 7, a pair of input terminals IMOand IIQI for receiving horizontal synchronizing-signal pulses fromhorizontal oscillator 59 of the receiver of Figure 2 are connected inthe input circuit of an electron-discharge device I I42. Terminal IIl-ii is connected to the control electrode H43 of device H42, and inputterminal II4I is.

grounded. The cathode II I I of device H42 is connected to groundthrough a cathode resistor I I 35. The anode I W5 of device I I42 isconnected to the positive terminal of a source I M'I of unidirectionaloperating potential, the negative terminal of which is grounded.

Anode M46 is coupled to the control electrode II IB of anelectron-discharge device H49 by means of a coupling condenser H50. Thecathode II5I of device H49 is directly connected to cathode II M ofdevice II lZ. Control electrode II 68 is connected to cathode II5Ithrough a variable resistor H52 and a fixed resistor H53 connected inseries. The anode H54 of device I I45 is connected to the positiveterminal of source I I4? through a load resistor I I55.

Anode I I54 of device I its is coupled to the control electrode H55 ofan electron-discharge device II5'I by means of a coupling condenser II58. Control electrode II 55 is connected to ground through a gridresistor I I59. The cathode I IBII of device H5! is connected to groundthrough a cathode resistor IItI. The anode H82 of device N51 isconnected to the positive terminal of source II4'I. M I

Cathode I I53 of device IE5? is directly coupled to the cathode I I63 ofanother electron-discharge device H54, thecontrol electrode H65 of whichis connected to ground through a resistor I III-6 and a source IIS'I ofnegative unidirectional biasing voltage, here shown as a battery.Control electrode H65 is also connected to input terminal II i through avariable resistor M68. The anode H69 of device [IE4 is connected to thepositive terminal of source II II through a load resistor The incomingcoded composite television signal is applied to a pair of inputterminals NH and H12 connected in the input circuit of an additionalelectron-discharge device III3. Terminal I III is coupled to the controlelectrode II'IlI of device II 13 through a couplin condenser H15.Terminal II'I2 is directly connected to ground. The cathode II'IB ofdevice II I3 is grounded. Control electrode I III of device I I73 isconnected to ground through a grid resistor I III. The anode II'IB ofdevice II'I3 is connected to the positive terminal or" source IM'I'through load resistor II'IO,

Output terminals II'IBand II 353 are provided for deriving a decodedsignal from the circuit of Figure 7. Terminal III9 is connected to anodeII'I8 of device Hi3, and terminal IIIIII is grounded.

The operation of the circuit of Figure 7 may best be described in.connection with the waveforms of Figure 6. Devices H42 and I I49 andtheir associated circuit components serve to manufacture horizontalblanking voltage pulses from the horizontal synchronizing-signal pulsesapplied between terminals H 25 and I I II from the syn- 11 I chronizingsig-nal separator of the receiver of Figure 2. The'duration' of theindividual blanking voltage pulses may be adjusted to the appropriatevalue by means of variable resistor I I52. Thus, synchronized horizontalblanking voltage pulses ,.represented by curve F of Figure 6 appear atthe control electrode I I56 of device I I51.

Electron-discharge device H64 is biased to cut off at a voltage E1intermediate the maximum positive-andnegative potentials H3I'and H36respectively of the control signal of curve E; Thus, when the controlsignal is at its maximum negative potential H36, during intervals ofnormal reception, device H64 is rendered non-conductive, and nocomponent of the current in load resistor H10 is due to the anodecurrent'of device H64. In such a condition, coded composite televisionsignals applied between terminals H1! and H12 by way of leads 54 fromthe video detector 43 (Figure 2) are translated by device H13 and appear in unaltered amplitude relation across load resistor I I10 andhence between output terminals I'I19 and H80.

When the control signal is at its maximum positive potential H3I, thevoltage developed across resistor H66 tends to counteract the voltage ofsource H61, and places electron-discharge device I I 64 under thecontrol of the horizontal blanking voltage pulses impressed on controlelectrode I I56 of device I I51. Device I I51 is biased to cut off at avoltage E2 (curve F) intermediate the maximum positive and negativepotentials I I32 and I I33 of the blanking'voltage pulses. Thus, in thepresence of a 'liorizontalblanking voltage pulse,

such as pulse H32 of curve F, device I I51 is rendered conductive, andthe potential of cathode H63 of-device H64 is raised, thus maintainingdevice H64 in'a non-conductive state and allowing the timing-signalcomponents of the incoming coded composite television signal to appearin unaltered amplitude relation between" output terminals H19 and H86.'Howevenduring intervals between blanking voltage pulses such asinterval H33, device I I51 is rendered non-conductive, and the potentialof cathode I I63 of device I I64 drops. This renders device I I64conductive and generates a voltage component I85 of curve K in resistorH10. Thus, during video-frequency signal intervals of the codedcomposite television signal, when the control signal is at itsmaximum'positive'potential, a decoding signal (curve K) which iscomplementary to the coding signal of curve G utilized at thetransmitter, is effectively superimposed on the incoming coded signal,and the output appearing betweenterminals m9 and use is efiectivelydecoded to produce the waveform shown in curve H.

The description of the construction and operation of the illustratedsystem has been predicated on the assumption that the coded compositetelevision signal is not subjected to undesirable fading in radiationfrom the transmitter to the receiver. In practice, it may be desirableto insert an automatic gain control amplifier (not shown) betweenterminal H4 and resistor H68 to control the. potential of controlelectrode H65 of device H64.in accordance with an automatic gain controlvoltage derived from video detector d3 (Figure 2) v The system providedby the present invention enables the transmitting station to have aneffective control over the distribution of thebro'ad cast information.This control may be achieved through the'agency of a central telephoneswitch board over existing telephone lines, or through the agency ofpower distribution networkspres ently in existence. Alternatively,aprivate branch exchange may be setf up-at' the transmitter to handlethe distribution'of thekey signaland' the attendant billing ofsubscribers utilizing the key signals.

The effect of-f'co ding -thebmnposite television signal in ac'cord'an'ceWith the present invention is tofplace on the airla signal which, whenreceived by a conventional non"-subscription type receiver or' by asubscription-typereceiverin the absence of an appropriatekey signal; ischaracterized by a variation in the-shade values of the reproduced imagewhichmanifests-itself as anex-t'remely obj'ectionable flicke'r; Thus, itis highly desirable from thelsubscriberspointoi? view-to obtain theproper key signal when he wishes to view the telecast.

While, in thee jbodimentshown and de scribed, the coding signalcomprises pulses of Q one polarity only and of fixedamplitude, it isparent that various changes and modifications may be made, and it istherefore contemplated in the appended claims to cover all such changesand modifications as fall within the true spirit and scope of theinvention.

We claim:

1, A subscription: type. .of television system comprising, incombination: a video-frequency signal generator; 'a scanning system forcontrolling said generator to develop during recurrent trace intervalsvideo-frequency signals rep resenting a scannedsubject and having apredetermined maximum amplitude-range and including a timing-signalgenerator for developing during interposed intervals timingsignalsnormally having a fixed amplitude with respect to said range; a mixerdevice coupled to said video.- frequency generator and to said scanningsystem for producing a composite television signal which includesinflalternation said videofrequency signals and said timing signals;coding apparatus "coupled to said mixer device for developing a codingsignal to alter the amplitude relation between said range. and theamplitude of said timing signals during spaced time intervals onlythereby to codesaid. composite signal; a key-signal-generator fordeveloping a key signal indicating the times of occurrence of saidspaced time intervals; a subscriber receiver including animage-reproducing device and an associated scanning system; means forradiating said coded composite signal tosaid receiver; a line circuitcoupled to said key-signalgenerator and extending to said receiver forsupplying said key signal to "said receiver; and decoding apparatusincluded insaid receiver and responsive to said key signal to develop adecoding signal which acts with said coded composite signal to causesaid reproducing device and its scanning 13 system to trace an image ofsaid scanned'sub- Ject.

2. A subscription type or" television system comprising, in combination:a video-frequency signal generator; a scanning system for controllingsaid generator to develop during recurrent trace intervalsvideo-frequency signals representing a scanned subject and having apredetermined maximum amplitude-range and including a timing-signalgenerator for developing during interposed intervals timing signalsnormally having a fixed amplitude with respect to said range; a mixerdevice coupled to said video-frequency generator and to said scanningsystem -for producing a composite television signal which includes inalternation said video-frequency signals and said timing signals; codingapparatus coupled to said mixer device and controlled by said scanningsystem for developing a coding signal to alter the amplitude relationbetween said range and the amplitude of said timing signals I duringspaced time intervals only thereby to code said composite signal; akey-signal generator controlled by said scanning system for developing akey signal indicating the times of occurrence of said spaced timeintervals; a subscriber receiver including an image-reproducing deviceand an associated scanning system; means for radiating said codedcomposite signal to said receiver; a line circuit coupled to saidkey-signal generator and extending to said receiver for supplying saidkey signal to said receiver; and decoding apparatus included in saidreceiver and responsive to said key signal to develop a decoding signalwhich acts with said coded composite signal to cause said reproducingdevice and its scanning system to trace an image of said scannedsubject.

3. A subscription type of television sysem comprising, in combination: avideo-frequency signal generator; a scanning system for controlling saidgenerator to develop during recurrent trace intervals video-frequencysignals representing a scanned subject and having a predeterminedmaximum amplitude-range and including a timing-signal generator fordeveloping during interposed intervals timing signals normally having afixed amplitude with respect to said range; a mixer device coupled tosaid video-frequency generator and to said scanning system for producinga composite television signal which includes in alternation saidvideo-frequency signals and said timing signals; a key-signal generatorcontrolled by said timing-signal generator; coding apparatus controlledconjointly by said key'signal generator and said timing-signal generatorfor developing a coding signal to alter the amplitude relation betweensaid range and the amplitude of said timing signals during spaced timeintervals only thereby to code said composite signal; a subscriberreceiver including an image-reproducing device and an associatedscanning system; means for radiating said coded composite signal to saidreceiver; a line circuit coupledto said key-signal generator andextending to said receiver for supplying to said receiver a key signalindicating the times of occurrence of said spaced intervals; anddecoding apparatus included in said receiver and responsive to said keysignal to develop a decoding signal which acts with said coded compositesignal to cause said reproducing device and its scanning system to tracean image of said scanned subject. 3

4. A subscription type of television system 14 comprising, incombination: a video-frequency signal generator; a scanning system forcontrolling said generator to develop during recurrent trace intervalsvideo-frequency signals representing a scanned subject and having apredetermined maximum amplitude-range and including a timing-signalgenerator for developing during interposed intervals timing signals ofconstant amplitude; a mixer device coupled to said videofrequencygenerator and to said scanning system for producing a compositetelevision signal which includes in alternation said video-frequencysignals and said timing signals; a key-signal generatorcontrolled bysaid timing-signal generator; coding apparatus coupled to said mixerdevice and controlled conjointly by said key signal generator and saidtiming-signal generator to shift said range with respect to saidconstant amplitude during spaced timed intervals only thereby to codesaid composite signal; a subscriber receiver including animage-reproducing device and an associated scanning system; means forradiating said coded composite signal to said receiver; a line circuitcoupled to said key-signal generator and extending to said receiver forsupplying tosaid receiver a key signalindicating the times of occurrenceof said spaced inter vals; and decoding apparatus included in saidreceiver and responsive to said key signal to develop a decoding signalwhich acts with said coded composite signal to cause said reproducingdevice and its scanning system to trace an image of said scannedsubject.

5. A subscription type of television system comprising, in combination:a video-frequency signal generator; a scanning system for controllingsaid generator to develop during recurrent trace intervalsvideo-frequency signals representing a scannedsubject and having apredetermined maximum amplitude-range and including a timing-signalgenerator for developing during interposed intervals timing signals ofconstant amplitude; a mixer device coupled to said video frequencygenerator and to said scanning system for producing a compositetelevision signal which includes in alternation said video-frequencysignals and said timing signals; akey-signal generator controlled bysaid timing-signal generator; coding apparatus coupled to said mixerdevice and conjointly controlled by said keysignal generator and saidtiming-signal generator for superimposing only during spaced timeintervals a direct voltage bias on said composite signal during saidtrace intervals only, thereby to code said composite signal; asubscriber receiver including an image-reproducing device and anassociated scanning system; means for radiating said coded compositesignal to said receiver; a line circuit coupled to said key-signalgenerator and extending to said receiver for supplying to said receivera key signal representing the times of occurrence ofsaid spacedintervals, and decoding apparatus included in said receiver andresponsive to said key signal to develop a decoding signal which iscomplementary to said coding signal and which acts with said codedcomposite signal to cause said reproducing device and its scanningsystem to trace an image of said scanned subject.

6. A subscription type of television receiver comprising: a first inputcircuit for receiving a composite television signal coded in accordancewith a coding signal, said coded composite signal includingvideo-frequency signals representing a scanned subject and having apredetermined 15 maximum ampl tudes-ease nd nt rposedtimns. ign ls havng 1 .m lifw q w ich s l er relative to, said range. only during spacedtime intervals by superposition. of said coding. signal;- asecond inputcircuit for receiving a key. signal indicatingthe times of occurrencevoi said? spaced intervals; an image-reproducing device and an associatedscanning system;v a control. circuit a coupled to said second inputcircuitand-responsive to said key signal for producing a. decodingsignal having an amplitude which. is complementary to th t. ot eid.eding" igna decoding apparatus coupled to saidfirst input circuit and to'sjaid'control circuit for superposing' said. de-

coding signal on said coded composite. signal to providea decodedcomposite video, signal; and

means for'utilizingsaid decoded compositevideo si nal to c se an reprodui i a its scanning system to. trace an image of said scanned subject 7.A subscription type of television, receiver comprising; .a, fir st inputcircuit for receiving. a composite television signal coded in accordancewith a codingsignal, saidcoded composite-signal including timing signalshaving a. constant, am.-

plitude and interposed video-frequency signals representing a. scannedsupjectand having apredetermined maximum amplitude-range which isshifted relative to saidconstant amplitude, only during spaced timeintervals by superposition ;of

said coding signal; a. secondfinput, circuit. for receiving a keysignal. indicating the times, of occurrence of said spaced intervals; animagereproducing device and an associated, scanning system; a controlcireuitcoupled. to.- said'. first and "second I input circuits-and .conjointly, responsive to; said key signal and to .said. timing signals forproducing a-decoding. signal having anamplitude which iscqmplernentarytothat ofi'said coding signal; decodingg apparatus coupled to said firstinput circuit and to. said control circuit for superposing; said.decoding, signal onv said coded composite signal to provideadecoded-jcomposite video signal; and meansifor utilizing said decodedcomposite; video, signal to. cause. said reproducing device andv its.scanning ,system: to trace animage of said seannedsubject.

8. In a subscriptiontypetelevision. system: a source ofcompositevideovsignalsincluding. videofrequency signal components and.interspersed timing-signal components; a source of. key signals having atiming characteristic. incommensurate with that of saidtiming-signal.-components; a

' timing-signal components; a source ofkey. signals having a timing.characteristic incommensurate withthat of. saidtiming-signal.components; a source. of periodic signals in synchronismwith said timingrsignal components; a .pulse generating coincidence.circuit coupled to saidkey-signal source and to said. periodic-signal.source 'and responsive to time coincidence of saidkey signals control;pulse-signal; and a. pulse generator, in-

.16 and said-periodic signals to produce acontrol pulse-signal; andmeans coupled to said coincidence circuit and to said compositevideosignalsouree and responsive to-said controlpulsesignal fordevelopinga modifying signal and for superposing said modif-ying signalonsaid composite" video signals to. alter the amplitude relationbetweensaid timing-signal components and said video-frequency components ofsaid composite video, signals. U 10; A subscription: type televisionreceiver com- I prising: asourceof coded composite video signalsincluding; alternately in time sequence. video;

frequencyco-mponents and timingsignal components, the amplitude relationbetween which componentsisvaried from time-totime between two. diiierentmodes a sourceofkeysignals indicating the times 0t transitionfrom oneofsaid modes tdthe other;v a source of periodic signals in; synchronismwith said timing-signal components,'-. a decoding device comprising apulse generating. coincidence circuit coupled to said key-signal sourceand to said periodic-signal source and, responsive to time coincidenceof saidizeysignals and said periodic signals to producesa 1 control:pulse-signal and means, included insaid decoding device, coupled to'saidcomposite-video-signal; source andto said coincidence. circuitandresponsive to said-control pulse-signal for developing a decodingsignal and for superposing. said decoding signal on said compositevideo. signals to 'alterthe amplitude, relation between saidtiming-signal components and said video-frequency components during.vintervals of reception inone of saidmodes;

. 11. A subscription type television receiver comprising: asourceofcoded composite video signals including alternately in time sequence:videofrequency components and timing-signal components, the amplituderelation between which components-isvaried fromtime to time'betweentwodifferen-t modes; a source of key signals indieating the times oftransition from one of said modes to;thc other; a source of periodicsignals in synchronismwith; said timing-signal components'ra decodingdevice comprising a pulse gencrating coincidence circuit coupledto saidkeysignalsource andto saidperiodic-signal source and responsive to timecoincidence'of" said key. signals and. said periodicsignals: to producea cludedcin: said decoding; device; coupled to.v said composite'video-signal source andlt'o' saidxcoin ,cidence circuit and. responsive:tozsaid control pulse-esignalrior. developing. a decoding signal duringintervals. of reception in .one'ofisaid modes andzfor superposing said,decodings signal" on said coded 1 composite xvideo-"signals;

FRANCIS. W: CROTIY; 'ERWINM: ROSCHKE;

; E EREN S-omen The followingv references are of record 'in the file. ofthis patentzi Y UNiTED-sTATEs-PATENTs Number;

Name Date I 2,236,502 I Goldsmith Apr. 1', 1941 22 5 15 5. R n hal'--.-u 5; 1941 $305,902" Schroeder Dec.22.,' 1942 '2,-s91;090 'Goldsmith Dec.18,1945 2,402,067 Mathes. June'll', 1946 2,403,059"; Dillenbeck July2,1946 2,405,252 Goldsmith .Aug. 6;,19'46 2,414,101 Hogan Jan.{14, 19472,472,774 Mayle June 7; 1949

